The present invention relates to an ultrahigh-speed clock extraction circuit that generates a local optical pulse stream or local electrical clock synchronized in bit phase with an ultrahigh-speed optical signal pulse stream of a repetition frequency over 100 Gbit/s which is input from the transmission line.
An increase in data traffic with the recent rapid widespread proliferation of the Internet is accelerating the implementation of larger-capacity optical communication networks. Along with wavelength multiplexing, time-division multiplexing is an effective technique for increasing the channel capacity or rate of each optical fiber, but the channel rate now achieved by speedups of electronic circuits is as high as 40 Gbit/s and further speedups are not easy. Optical signal processing, which utilizes the nonlinear optical effect that provides a response on the order of subpico second, is expected to overcome the band limitations on electronic circuits and is now under study and development with a view towards active application to optical communications.
The optical signal processing based on the nonlinear optical effect is to cause timed interaction between the received optical signal and a locally generated optical signal (locally generated optical control signal) to perform various signal processing operations, such as switching, wavelength conversion and so forth, in the domain of light. The required accuracy of timing for the interaction increases with faster optical signal processing, for example, 1 ps or below for 100 Gbit/s signal processing. In optical communications, the optical signal usually propagates through optical fibers over a long distance, and consequently, the timing of arrival of the optical signal at the receiving side varies due to expansion and contraction of the optical fibers. Accordingly, identification of each bit of the received optical signal usually calls for extraction of a clock corresponding to the varying timing of arrival at the receiving side. To perform the optical signal processing, it is necessary that the receiving side generate, as the local clock, a local optical control pulse stream of the varying timing of the received optical signal. The optical control pulse stream is generated, in general, by a mode-locked laser or similar short-pulse laser and subjected to amplification and other processing by an optical fiber amplifier and is then coupled or combined with the received optical signal; in this instance, a propagation delay in the processing circuit itself, which is caused by temperature-dependent expansion or contraction of the optical fiber forming it, readily varies with ambient temperature at a rate of approximately 50 ps/km/° C., constituting a critical factor that prevents appropriate optical signal processing.
FIG. 1 is a block diagram depicting an example of a conventional ultrahigh-speed clock extraction circuit described in European Patent Application Publication EP 1119119A 1 (corresponding US Patent Application Publication Gazette US2002/0027692A1).
The illustrated ultrahigh-speed clock extraction circuit comprises: a local clock generating part 20 for generating a local clock CKL; a local feedback signal generating 30 for generating a local feedback signal FBL; an input signal component generating part 40 for extracting an input signal component SCR from an input optical signal pulse stream LPR; and a phase comparison part 50 which compares the phases of the local feedback signal FBL and the input signal component SCR and provides the comparison result, as a voltage control signal VC, to the local clock generating part 20. The local clock generating part 20, the local feedback signal generating part 30 and the phase comparison part 50 constitute a phase-locked loop.
The local clock generating part 20 is made up of a voltage-controlled oscillator 21, a local pulse generating light source 22, and an optical branching device 23. The input signal component generating part 40 is made up of an optical modulator 41, a photodetector 42 and a multiplexer 43. In the accompanying drawings, symbols in each pair of brackets [**] are intended to represent a frequency, and thick solid lines indicate optical signal paths and thin solid lines electrical signal paths.
The input optical signal pulse stream LPR of a repetition frequency Nfa (where N is the number of multiplex channels), sent over an optical fiber transmission line 9, is branched by an optical branching device 11 into two, one of which is output as an original input optical signal pulse stream LPR and the other of which is fed to the optical modulator 41 of the input signal component generating part 40. On the other hand, the local pulse generating light source 22 is driven by a drive signal SD of a frequency fVCO from the voltage-controlled oscillator 21 to generate a local optical pulse stream LPL. The local optical pulse stream LPL is branched by the optical branching device 23 into two, one of which is output as the local clock CKL via a local output path 2LP and the other of which is fed via a feedback path 2FP to the photodetector 31 for conversion into the local feedback electrical signal FBL.
The local feedback signal FBL of the frequency fVCO from the photodetector 31 is branched into two, one of which is L-multiplied by the multiplier 43 to a modulation signal Sm of a frequency fm=LfVCO, which is applied to the optical modulator 41. The optical modulator 41 is one that has a property of linearly responding to the electrical drive signal, for example, an electroabsorption optical modulator.
The input signal component SCR of a frequency Nfa−n(LfVCO) (where n is a natural number) is extracted by the photodetector 42 from the optical signal output from the optical modulator 41, and the extracted signal is provided to a phase comparator 51. The electrical feedback signal FBL branched from the photodetector 31 is M-multiplied by a multiplexer 52 into a signal FBL of a frequency MfVCO, which is applied to the phase comparator 51 for comparison with the input signal component SCR of the frequency Nfa−n(LfVCO). An error signal resulting from the comparison is fed back as the voltage control signal VC to the voltage-controlled oscillator 21 to control its oscillation frequency fVCO. As a result, the local optical pulse stream LPL from the local pulse generating light source 22, which is output from the optical branching device 23, is provided from the local output path 2LP as the local clock CKL synchronized in bit phase with the input optical signal pulse stream LPR.
Referring next to FIGS. 2A and 2B, the function of the optical modulator 41 will be described.
FIG. 2A depicts the frequency spectrum of the output light from the optical modulator 41 when an optical pulse stream of a repetition frequency f0 is modulated therein by the modulation signal Sm of the frequency fm. The optical pulse stream of the repetition frequency f0 has modulated components fc+f0 and fc−f0 on both sides of an optical carrier frequency fc. Modulating the optical pulse stream by the electrical signal Sm of the frequency fm in the optical modulator 41, many modulated sidebands are newly generated by the frequency fm around the carrier component fc and the modulated components fc±fo, respectively, as shown in FIG. 2A. By converting such a modulated signal in the photodetector 42 into an electrical signal, beat signals (fc±nfm, where n is a natural number) appear in its power spectrum at intervals fm about the repetition frequency f0 as depicted in FIG. 2B. Even with the actual photodiode of limited frequency band, it is possible to detect beat signals in the low-frequency region.
The modulated sidebands hold phase information of the original signal, and the phase information is also reflected in the beat between the modulated sidebands. Accordingly, by forming a phase-locked loop through use of the beat signal that is produced by applying the modulation signal Sm of the frequency fm=LfVCO from the multiplexer 43 to the optical modulator 41, it is possible to generate an electrical signal synchronized with an optical pulse stream of a repetition frequency over 100 GHz.
Consider, for example, the case where Nfa=160 GHz, fVCO=20 GHz and L=M=2 in the conventional ultrahigh-speed clock extraction circuit depicted in FIG. 1. This case corresponds to the generation of an optical control pulse stream (i.e., local optical pulse stream LPL) synchronized in bit phase with a time-division-multiplexed optical signal (i.e., input optical pulse stream LPR) of a repetition frequency 160 GHz and having a repetition frequency 20 GHz. The modulation frequency of the optical modulator 41 is fm=LfVCO=40 GHz, and the beat signal frequencies that are observed at the output of the photodetector 42 are 160−n×40=120, 80 and 40 (GHz) for n=1, 2 and 3, respectively. From the viewpoint of frequency bands possible with electric circuits, 40 GHz is appropriate for phase comparison use. The setting M=2 for the multiplier 52 in the phase comparison part 50 corresponds to the phase comparison frequency 40 GHz.
The above condition for operation raises such a problem as described below. That is, the beat signal at the output of the photodetector 42 contains not only the beat (first beat components 160−n×40, where n=0, 1, 2, 3, . . . ) between the 160 GHz component of the input optical signal and the modulation frequency (fm=LfVCO=40 GHz) component of the optical modulator 41 but also a beat between the DC (0 Hz) component of the input optical signal and the modulation frequency fm (40 GHz) component of the optical modulator 41; namely, the output signal from the photodetector 42 contains, as a second beat component, components of the frequency fm of the modulation signal Sm and a frequency kfm (where k=1, 2, . . . ) that is a natural-number k multiple of the frequency fm. Thus, the first and second beat components both contain exactly the same frequency 40 GHz. As referred to previously, the first beat component of the frequency 160−nfm has the phase information of the input optical signal and is utilized in the phase-locked loop, but the second beat components kfm (frequencies 40, 80, . . . GHz) do not contain the phase information of the input optical signal. Hence, the presence of the second beat components interferes with the phase comparison by the phase comparator 51 between the first beat component and the output component MfVCO from the multiplexer 52, making the entire operation of the phase-locked loop unstable.
Further, for example, when L=3, M=2 and n=2, then fm=60 GHz; if the 40 GHz component is used as the first beat component (100, 40, . . . (GHz)), it is possible to avoid the interference by the second beat component (60, 120, . . . (GHz)) at the phase comparison frequency 40 GHz. In other words, it is possible to avoid that the first and second beat components contain the same frequency. This requires, however, the use of fm=60 GHz (and 40 GHz as the phase comparison frequency) as the modulation frequency; in the present microwave technology, this scheme is appreciably disadvantageous in terms of cost and performance as compared with the case of using frequencies down to the K band lower than 26.5 GHz.
In short, any combinations of the parameters L, M and n neither permit cost reduction nor provide stability in the generation of the local clock CKL synchronized in bit phase with the 160 Gbit/s optical time-division-multiplexed signal and having the 20 GHz repetition frequency.